The column referenced in the jspx file,

<f:facet name="header">
<ice:commandSortHeader
columnName="DEF"
arrow="true" >
<ice:outputLabel value="D-E-F" />
</ice:commandSortHeader>
</f:facet>

must much the column name in the backing bean,

xtable = new CheckBoxTable();
...
xtable.setColumnNames(new String[] { "ABC", "DEF"});